This invention relates to a shield structure for a vacuum switching circuit interrupter.
FIG. 1 is a sectional view showing the structure of a conventional vacuum circuit interrupter as disclosed in Japanese Utility Model Publication No. 53-43491, for example.
In the figure, the vacuum circuit interrupter comprises an electrically insulating cylinder made of a glass or a ceramic material. A first flange 4 is attached to the upper end of the insulating tube 1 through a cylindrical sealing member 3, and a second flange 6 is attached to the lower end of the insulating tube 1 through a cylindrical sealing member 5. The first flange 4 has secured at its center a stationary electrode rod 8 having a stationary electrode 7 at its lower end, and the second flange 6 has secured at its center an axially expandable bellows 9, and the other end of the bellows 9 has mounted thereon a movable electrode rod 11 having at its tip a movable electrode 10 opposing the stationary electrode 7. The electrode rods 8 and 11 are axially aligned, and the insulating tube 1, the sealing members 3 and 5, the flanges 4 and 6, and the bellows 9 together constitute a vacuum vessel 12. To the insulating cylinder 1 a central portion of a cylindrical main shield 13 of a circular cross-section and suitably curved to surround the electrodes 7 and 10 to have a diameter smaller at the opposite ends than that of the central portion is mounted. Also, on the inner surface of the first flange 4 an outer shield 14 is provided, and the lower end of the outer shield 14 is formed to concentrically overlap the upper end of the main shield 13 and be radially spaced from the outside through by a suitable gap therebetween. Also on the upper surface of the second flange 6 an outer shield 15 is provided and the uppeer end of of outer shield 15 and the lower end of the main shield 13 are formed in a relationship similar to the above. Further, a bellows shield 16 surrounding the bellows 9 is mounted to the movable electrode rod 11.
The operation of the conventional vacuum circuit interrupter will now be described. When the electrodes 7 and 10 are opened while an electric current flows through the electrode rods 8 and 11, an electric arc is generated across the electrodes 7 and 10.
This arc melts the electrodes 7 and 10 and generates metal vapor which is allowed to diffuse into the vacuum space. In order to prevent pollution of the insulating vessel 1 by the metal vapor, the main shield 13 is provided thereby to trap most of the metal vapor.
This phenomenon occurs when the space between the electrodes 7 and 10 and the main shield 13 is large, and when the vacuum interrupter is very compact the arc generated across the electrodes 7 and 10 is driven to the outer periphery of the electrodes 7 and 10 by a magnetic field generated by the arc, often causing the main shield 13 to melt.
Since the conventional vacuum interrupter is constructed as described above, particles or small fragments of the melted main shield 13 are scattered in the axial direction of the main shield 13 and, after they reach the curved portions, they also scatter and condense in the radial direction. Therefore, the distances between the electrode 7 and the shield 13 as well as the electrode 10 and the shield 13 are shortened, decreasing the dielectric recovery characteristics during current interruption and the withstand voltage characteristics and after current interruption.